WASHINGTON (AP) — Newly discovered fossils give scientists the first real look at when Earth underwent a critical transition from plants and unrecognizable simple animals to the complex creatures that took over the world and eventually led to us humans.
It happened millions of years earlier than researchers thought.
Researchers say more than 700 fossils discovered in southwestern China’s Yunnan province offer a window into life 539 million years ago, when animals at the end of the Ediacaran Period were simple but strange, living two-dimensionally in the ocean, never rising or falling.
But a study published Thursday in the journal Science said many of the fossils in the hoard are remnants of more complex animals that lived three-dimensional lives, traveling in water and eating. These features are thought to have emerged only at least 4 million years later during the Cambrian period, the so-called Cambrian explosion of complex and identifiable animal life.
“This is really our first window into how the modern animal-dominated biosphere basically formed and developed and went through this strange Ediacaran transition,” said co-author Frankie Dunn, a paleontologist at the University of Oxford’s Museum of Natural History. “We come from a two-dimensional world where, in the blink of an eye of geology, animals have diversified. They’re everywhere. They’re doing everything, they’re changing biogeochemical cycles. They’ve changed the world.”
The new finds are not far from the U.N. Chengjiang World Natural Heritage Site, where other fossils exposed by the roadside are less glamorous but have different layers “where you can really travel through time, geologic time, in the landscape,” Dunn said. One area provides a “snapshot” of the forces evolution has brought together.
Complex animals developed with symmetry
At that location, Dunn said, the group of fossils includes examples of bizarre life that existed early but disappeared, as well as early examples of organisms that would evolve into modern animals. Importantly for those more modern animals, the left and right sides of their bodies are essentially the same.
Now, almost all animals on Earth have similar features on the left and right sides, as well as heads and anuses. Before the fossils were discovered in China, scientists had seen traces of this symmetrical body shape in fossil footprints, but not the creatures themselves.
“Now we know what they were formed from because we have these fossils for the first time,” said study co-author Ross Anderson of the Oxford Museum of Natural History.
Help resolve the “Rock vs. Clock” debate
To date, the field of paleontology has been conflicted. Genetic analyzes of trait mutations and rates of evolution suggest humans and starfish had an earliest common ancestor during the Ediacaran period, but there’s no fossil or rock indication that it happened, Dunn said. It’s called the “rock versus the clock” debate, she said.
“Our new fossil site tells us that the rocks and clocks may actually be closer in agreement than we thought,” Dunn said.
Emily Mitchell, a paleontologist at the University of Cambridge who was not involved in the study, said the new research is “significant because the Ediacaran contained animals, and we know there must have been a transitional phase between them and the Cambrian fauna. But so far we have no real evidence for that.”
Some outside scientists, such as Jonathan Antcliffe of the University of Lausanne, question whether there is enough evidence to call the fossils complex animals, but most experts contacted by The Associated Press think so.
Trying to figure out how and why
Now that scientists know when the explosion of life occurred, they have more questions and a few theories.
“I really wanted to understand, not just when it happened, which is interesting, but how it happened and why it happened the way it did,” Dunn said. “So, can we disentangle the feedbacks between Earth and life, or between life and life? Once Ediacaran exists on the seafloor, is something close to the Cambrian explosion inevitable? I find these questions very interesting.”
Life on Earth began 3 billion years ago, but it took another 2.4 billion years for complex animals to appear. They then multiplied, diversified and quickly became dominant, Dunn said.
Charles Marshall, a paleontologist at the University of California, Berkeley, who was not involved in the study, said this may be because the Earth had to accumulate high enough oxygen levels and evolution had to kick-start genetic changes.
“The Cambrian explosion was sudden because there were already rich developmental systems in place,” Marshall said.
“The fundamental change that occurred during this period was the way animals on Earth interacted with each other,” said Duncan Murdock, director of the Oxford Museum, where many of the authors worked. “Once animals came along and started eating each other and churning up sediments, they changed the Earth forever. And the planet we live on is very much built on the Ediacaran and Cambrian periods.”
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Associated Press writer Siobhan Starrs contributed from London.
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